This series and parallel sensor circuit was found in Popular Electronics from the 1990s. Any SCR can be used and the supply voltage depends on the load.
This circuit works as both an injector and a signal follower. We found it in the May 1994 issue of...
This midrange filter for sound systems was found in the May 1988 issue of Radio Electronics...
An OR gate can be improvised using NAND gates of a 4001 as shown here. The diodes are...